1. Field of the Invention
The present invention relates to a display device and more particularly to a technology effectively applicable to a display device with a TFT element having an amorphous semiconductor and a TFT element having a polycrystalline semiconductor both formed on the same substrate.
2. Description of the Related Art
Conventionally, display devices include a liquid crystal display device having a liquid crystal display panel with a liquid crystal material encapsulated between a pair of substrates. A liquid crystal display device is used for a monitor (display) of a TV or PC, for example. For the liquid crystal display device of a TV or the like, an active matrix type liquid crystal display panel (hereinafter simply referred to as “liquid crystal display panel”) is used.
In the liquid crystal display panel, a display region is set by an assembly of a plurality of pixels, and each of the pixels has a TFT element functioning as an active element (also referred to as “switching element”), a pixel electrode, and a counter electrode (also referred to as “common electrode”).
The TFT elements of the pixels are arranged in a matrix form on one of the pair of substrates (hereinafter referred to as “TFT substrate”). On the TFT substrate, a plurality of scanning signal lines, a plurality of video signal lines, and the pixel electrodes are arranged in addition to the TFT elements.
For an active layer of the TFT element (active element) of the liquid crystal display panel, an amorphous semiconductor such as of amorphous silicon has been conventionally used in many cases. In recent years, however, a polycrystalline semiconductor element such as of polycrystalline silicon has been increasingly used.
In a conventional liquid crystal display device, an IC chip that is manufactured in a separate step from the liquid crystal display panel has been used for a data driver or gate driver for driving the liquid crystal display panel, and is connected to the liquid crystal display panel in an assembly step of the liquid crystal display device in many cases. However, some liquid crystal display devices in recent years use a liquid crystal display panel in which a drive circuit having a function equivalent to that of the IC chip of the data driver or gate driver is formed simultaneously with the TFT elements (active elements), the scanning signal lines, the video signal lines or the like outside the display region of the TFT substrate in a manufacturing step of the TFT substrate, for example.
The drive circuit formed (incorporated) in the TFT substrate is an integrated circuit of a TFT element different from the active element, a capacitor element, a resistance element or the like, and must be operated at a high speed. Therefore, a polycrystalline semiconductor such as of polycrystalline silicon is desirably used for an active layer of a TFT element of the drive circuit.
Here, there are two cases: One is that the TFT element of the drive circuit and the TFT element (active element) of the display region both have an active layer of a polycrystalline semiconductor; and the other is that the TFT element of the drive circuit has an active layer of a polycrystalline semiconductor, and the TFT element of the display region has an active layer of an amorphous semiconductor.
In the case where, among conventional TFT substrates, a TFT substrate in which an active layer of the TFT element of the display region is formed of amorphous silicon is used to form the drive circuit outside the display region of the TFT substrate, the TFT element of the drive circuit has an active layer of a polycrystalline semiconductor, and the TFT element of the display region has an active layer of an amorphous semiconductor in many cases because a conventional manufacturing process of the TFT substrate is utilized.
As described above, when the TFT element having an active layer of a polycrystalline semiconductor (hereinafter referred to as “p-TFT element”) and the TFT element having an active layer of an amorphous semiconductor (hereinafter referred to as “a-TFT element”) are simultaneously formed on the surface of one insulating substrate, for example, an amorphous semiconductor film is formed on the entire surface of the insulating substrate, the amorphous semiconductor only in the region for forming the drive circuit is melted and crystallized to form a polycrystalline semiconductor, and thereafter the partially polycrystallized amorphous semiconductor film is etched to form active layers of the respective TFT elements. For melting the amorphous semiconductor film, for example, a method referred to as “excimer laser annealing” (ELA) is used (refer to JP-A-5-55570, for example).